Why We Grow Large and then Grow Old: Biology, Economics and Mortality
Introduction (Via Uchicago)
A basic mystery concerning complex life forms is why they develop, grow and improve during a first phase of life, only then to decay and senesce during a second phase. Indeed, two important related problems have plagued the evolutionary theory of aging. (This theory is developed in Hamilton, 1966, Kirkwood, 1990, Medawar, 1952, and Williams, 1957.) While these existing evolutionary models account for increasing mortality rates with age, they fail to account for the nearly universal decrease in mortality with age during the first phase of life among higher organisms. Second, this formal theory does not explain extended periods of survival after reproduction has ceased.
Central Idea (Excerpted Via Uchicago)
The basis of the theory is that organisms invest in somatic capital, which is then used to produce energy to support continued life and reproduction. Such somatic capital is characterized by both quantity and quality. The quantity of capital is the number of cells, which is closely related to mass. In most multi-cellular organisms, growth is a unidirectional process in which cell number increases with age, either throughout life in the case of indeterminate growth, or up to some age, in the case of determinate growth. Cell quality, as measured by functional efficiency, is allowed to vary and can be improved by investments in repair. Without such investments, cell quality depreciates over time due to environmental assaults and the build-up of deleterious by-products of cell metabolism.
….The cost of investment in quality depends on the quantity of capital, because each cell is subject to deterioration and has its own maintenance costs. Under plausible conditions, it is now evolutionarily optimal to generate a high level of initial quality, but then to let it fall with age….